It look like you already changed your protected htaccess credentials because i was unable to access your site and backend page today.

Also can you provide us one of the link how it redirect to 404 error after user login?

Hi Arlex,
please try to login to my staging server now.

Please view EasyDiscuss front end page here: https://beta.pohyby.co.uk/diskuse
There is a module on the right with "Ask a Question button", when I click there I get a login prompt and Iam redirected to 404 page when successfully logged in. (please use same credentials as for joomla backend access. (username support))

It seems like that is bug when you configure login provider to use Easysocial, I will log this issue into our issue tracker.

For now perhaps you can try temporary switch to "Joomla" login provider first, it should redirect to the correct page.